In recent years, reduction in thickness, reduction in weight, and flexibility are demanded for devices in accordance with rapid progress of displays such as a liquid crystal display, an organic EL display, and an electronic paper, and of electronics such as a solar cell and a touch panel. Under the circumstances, instead of a glass substrate which is used in such devices, a plastic film substrate has been studied which can achieve reduction in thickness, reduction in weight, and flexibility.
In the devices, various electron elements, e.g., a thin film transistor and a transparent electrode are provided on a substrate, and it is necessary to carry out a high temperature process for forming the electron elements. Therefore, the plastic film substrate is required to have heat resistance sufficient for being applied to the high temperature process. Moreover, in a case where the electron elements, which are made of inorganic materials, are formed on a film, the film on which the inorganic devices have been provided may be curved and further the inorganic devices may be broken, due to a difference in linear thermal expansion coefficient between the film and the inorganic materials. Therefore, a material has been demanded which has (i) heat resistance and (ii) a linear thermal expansion coefficient equivalent to that of the inorganic material.
In a case where light from a display element (such as liquid crystal or organic EL) is emitted through a plastic film substrate (e.g., in a case of a bottom emission type organic EL), the plastic film substrate needs to have transparency. In particular, the plastic film substrate is requested to have a high optical transmittance in a visible light range, i.e., in a wavelength range of 400 nm or less. In a case where light passes through a retardation film and/or a polarizing plate (e.g., in a case of a liquid crystal display or a touch panel), the plastic film substrate is required to have high optical isotropy, in addition to transparency.
Processes for producing such devices are classified into a batch type and a roll-to-roll type. In a case where the roll-to-roll production process is used, it is necessary to prepare new equipment, and also necessary to overcome some problems caused due to rotation and contact. Meanwhile, in the batch type process, a coating resin solution, which has been applied to a glass substrate, is dried so as to be formed into a substrate, and then the substrate thus obtained is peeled off. As such, in the batch type process, conventional equipment for processing a glass substrate such as a TFT can be used, and is therefore advantageous in terms of cost.
Under the circumstances, development of a material has been strongly demanded which (i) is applicable to a conventional batch process and (ii) has high heat resistance, low thermal expansion property, excellent transparency, and low birefringence.
As a material which satisfies the above described requests, a polyimide material has been studied which is known as a material having excellent heat resistance. In a case of preparing a polyimide that is high in transparency and is low in thermal expansion property, a monomer having a rigid structure and an alicyclic monomer are generally used (see Patent Literature 1 and Patent Literature 2). Meanwhile, it is known that a polyimide having a fluorene structure shows heat resistance and a low water-absorbing property (Patent Literature 3).